Add set-transmit-power documentation page

docs/reference/radio/set-transmit-power.md

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+# Set Transmit Power
+
+Sets the transmitter power for ``radio`` communications.    
+The power can be set to a value between 0 (-30dbm) and 7 (+4dbm).
+
+## Range
+   
+At power level 7, in an open area without significant interference (coming from WiFi networks or other devices operating on the 2.4 GHz range), you can get a **range of over 70m**.   
+ 
+Indoors (or with additional interference), range will be significantly reduced.
+
+## Important Security Consideration
+
+The functions in the ``radio`` namespace allow the BBC micro:bit to communicate with other micro:bits.
+
+This API does not contain any form of encryption, authentication or authorization. It's purpose is solely for use as a teaching aid to demonstrate how simple communications operates, and to provide a sandpit through which learning can take place.
+
+For serious applications, BLE should be considered a substantially more secure alternative.
+
+```sig
+radio.setTransmitPower(1)
+```
+
+### Parameters
+
+* ``power`` -- a [number](/reference/types/number) between ``0`` and ``7``.
+
+### Example
+
+Sets the transmitter power to full power at 7.
+
+```blocks
+radio.setTransmitPower(7)
+```
+
+### See also
+
+[receive number](/reference/radio/receive-number), [send number](/reference/radio/send-number), [on data received](/reference/radio/on-data-received)